The present invention relates to smoke detectors that use the principle of the decreased conductivity responsive to smoke conditions to provide an appropriate alarm.
The fundamental objective in the smoke detectors of recent development is to give an early warning of the presence of smoke that is indicative of an incipient fire. Only in this way can lives be saved by such preventive means; otherwise, because of the time period involved between the earliest indication of smoke an the actual outbreak of fire, lives can be needlessly lost because persons in a building or the like will be overcome before they are able to perceive that a dangerous condition exists.
Accordingly, major efforts have been directed to making smoke detectors ever more sensitive to low levels of smoke. Various operating principles have been employed to this end, such as the optical and ionization current techniques. It is with the latter technique that the present invention is concerned.
In order to provide background material for understanding of the ionization operating principle in smoke detectors and the like, reference may be made to the following U.S. Pat. Nos. 3,521,263; 3,559,196; 3,676,680; 3,710,110; 3,909,813. Of particular pertinence to the present invention is U.S. Pat. No. 2,994,768 in which there is described a system for determining the content of aerosols in a gas by means of measuring a unipolar current flowing in a gas discharge device.
Especially relevant to the present invention is a report from a National Research Council Symposium entitled "Fire Detection for Life Safety", held Mar. 31 and Apr. 1, 1973, such report bearing the title "Physical Aspects of Ionization Chamber Measuring Techniques (Unipolar and Bipolar Chambers)", the author being Andreas Scheidweiler, Cerberus, Ltd., Mannedorf, Switzerland, published in 1977. In that article an analysis is presented of the operation of ionization detectors and, in particular, of the more common, i.e., bipolar, ionization chambers and a presentation of what is termed a unipolar ionization chamber, the latter involving conditions imposed within the chamber such that interelectrode spacing is long, compared to the range of the ionizing rays, and only the immediate area in front of one electrode is ionized. Consequently, when an electric field is applied, by connection of a suitable source of potential to the electrodes, only ions of one sign emerge in the part of the chamber that is not ionized. The pairs of ions produced are separated by the field so that only unipolar ions emerge from the ionization zone, whereas in the ionization zone itself a bipolar ion current flows. Such a chamber, in which the conducting path includes a region having ions of only one polarity, is called a unipolar ionization chamber.
Accordingly, there are several advantages which appear to exist for the unipolar ionization chamber, namely, better smoke sensitivity seems to obtain. Also, the unipolar chamber appears to have greater stability, and there appears to be lower sensitivity to humidity variations and dust accumulation, while providing lower sensitivity to air currents. However, there are difficulties presented to developing a design or arrangement that will not involve excessive height for the chamber or chambers.
Accordingly, it is a primary object of the present invention to enable a smoke detector of unipolar design to be constructed within reasonable dimensions.
Another object is to provide a dual chamber detector operating on the unipolar principle and with very close spacing among the three electrodes required.
A further object is to insure that the detecting and reference chambers in the above-noted dual chamber detector have identical characteristics so that the detector operates with optimal cancellation of ambient effects.
The above and other objects are implemented and fulfilled by a primary feature of the present invention according to which specialized configurations and locations for the operating elements of an ionization smoke detector are provided. In brief, the provision of a unipolar ionization chamber for efficient detection of smoke in a detector of reasonable proportions is accomplished by a construction of that unipolar chamber such that the unipolar region can be developed in a way that involves a much smaller space for the total chamber. In other words, instead of a straight path or direct configuration for the positive and negative electrodes with respect to the ionization pattern, in accordance with applicant's invention these electrodes are specially configured and the ionizing source is selectively placed in a confronting relationship with one of the electrodes. The precise configurations will be described hereinafter in accordance with the more specific features of the present invention.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.